The long-term goal of this project is to optimize the use of alternative marrow donors for those patients with severe aplastic anemia who lack siblings with an identical HLA genotype. The main objective is to develop an effective strategy to induce immunological tolerance between recipient an donor that will allow engraftment of donor hematopoietic stem cells while preventing graft-versus-host disease. Donors will be HLA-compatible for one locus mismatched unrelated volunteers or haploidentical relatives. The first specific aim is to establish novel immunosuppressive regimens that will be sufficient to allow engraftment of donor hematopoietic stem cells from marrow or blood without the use of cytotoxic therapy or irradiation in patients with marrow failure. Recent studies under the auspices of this grant have indicated that immunosuppression with a T cell receptor /CD3 complex-specific monoclonal antibody named BC3 plus high dose glucocorticoids allows engraftment of second transplants in patients sensitized after rejection of a previous graft from the same donor. We propose to determine whether the same transplant protocol will be adequate to allow engraftment of a first unrelated donor transplant in patients with severe aplastic anemia. The second specific aim will determine whether donor-specific tolerance achieved by the CD3 protocol is associated with reduction in host anti-donor immune reactivity. Patients with the highest frequency of donor-specific natural killer cells or cytotoxic T lymphocytes are at highest risk for graft rejection. Samples of patient peripheral blood lymphocytes will be obtained before and after transplant and tested in limiting dilution assay for the frequency of cytolytic precursors specific for donor cells. Selected clones will be fully characterized. Results of these experiments may elucidate the mechanism of graft tolerance or rejection and point the way towards improved treatment regimens. The third aim is to determine whether new regimens of post-transplant immunosuppression can decrease the incidence of GVHD and raise the limits of acceptable donor HLA disparity in haploidentical recipients with severe aplastic anemia or other nonmalignant diseases.